I would add the Dwarf race.
If you think about it, you could dig up a bunch of units.
1. Dwarf Warrior (take from 3 heroes, for example)
2. Dwarf Archer (give him some kind of crossbow and let him blast away)
3. Dwarf on an Iron Dragon (from 4 heroes)
4. Dwarf Engineer (attacks walls) well, and shoots from a ballista, for instance, or put him on some kind of artillery piece.

And add a crafting skill to the heroes, for example, or looting—like after every battle there's a chance to strip an artifact or resources.....